1. Field of the Invention
The present invention relates to a process for preparing 4-aminobut-2-enolides and also corresponding intermediates and starting compounds which are passed through or used in the process according to the invention. The present invention further provides processes for preparing the corresponding intermediates and starting compounds.
2. Description of Related Art
Particular substituted 4-aminobut-2-enolide compounds are known as insecticidally active compounds from EP 0 539 588 A1. In addition, patent applications WO 2007/115644, WO 2007/115643 and WO 2007/115646 also describe corresponding insecticidally active 4-aminobut-2-enolide compounds.
In general, enaminocarbonyl compounds are synthesized from tetronic acid and an amine according to scheme 1 below. This procedure is described, for example, in EP 0 539 588 A1 and in Heterocycles Vol. 27, No. 8, pages 1907 to 1923 (1988).

A particular disadvantage of this process is that anhydrous tetronic acid is required as a starting material, the preparation of which is inconvenient and costly.
For instance, tetronic acid is generally prepared proceeding from ethyl acetoacetate via a bromination and subsequent hydrogenation (cf. Synthetic Communication, 11(5), pages 385 to 390 (1981)). The overall yield of tetronic acid proceeding from ethyl acetoacetate is less than 40%, which makes the process relatively unattractive from an industrial point of view.
CH Patent 503 722 describes a further process for preparing tetronic acid. In this process ethyl 4-chloroacetoacetate is reacted with an aromatic amine to give 3-arylaminocrotolactone and then the tetronic acid is released by treatment with mineral acids. The disadvantage of this process is that the isolation of the tetronic acid is possible only by high-vacuum sublimation, which makes this process too relatively unattractive from an industrial point of view.
A further process for preparing tetronic acid is described in EP 0 153 615 A, in which the starting materials are 2,4-dichloroacetoacetic esters. This likewise multistage and complicated process affords the desired compound likewise only with a moderate overall yield of 65%.
Tetrahedron Letters, No. 31, pages 2683 and 2684 (1974), describes the preparation of tetronic acid and of a corresponding enaminocarbonyl compound. The synthesis described there is reproduced in scheme 2 below. The reactant used is dimethyl acetylenedicarboxylate.

Disadvantages of this process are the low overall yield of 30% and the requirement to have to use costly reactants, for example lithium aluminium hydride (LiAlH4), as reagents.
Additionally known from the prior art is a process for preparing 4-aminobut-2-enolides proceeding from methyl tetronate (J. Heterocyclic Chem., 21, 1753 (1984)). For this process, the starting material used is the costly 4-bromo-3-methoxybut-3-enecarboxylic ester.
A further process proceeds from a 4-chloroacetoacetic ester, which is reacted with amines (Heterocycles, Vol. 27, No. 8, 1988, pages 1907 to 1923). The reaction to give the aminofuran is conducted in one step. In this step, the amine is added with glacial acetic acid to a solution of 4-chloroacetoacetic ester in benzene and the resulting mixture is heated under reflux for several hours. The yields of 4-methylamino-2(5H)-furanone in this synthesis are only 40%.
EP 0 123 095 A discloses a process in which tetronamide is prepared from 3-amino-4-acetoxycrotonic ester. 3-Amino-4-acetoxycrotonic ester is costly and inconvenient to prepare, and so an economically viable synthesis is not possible by this process.
A further process for preparing tetronic acid proceeding from malonic esters and chloroacetyl chloride is known from J. Chem, Soc., Perkin Trans. 1 (1972), No. 9/10, pages 1225 to 1231. This process affords the desired target compound with a yield of 43%.
The aforementioned international patent application WO 2007/115644 describes the preparation of 4-aminobut-2-enolides, for example of 4-[[(6-chloropyridin-3-yl)methyl](3,3-dichloroprop-2-en-1-yl)amino]furan-2(5H)-one, by reaction of 4-[[(6-chloropyridin-3-yl)methyl]amino]furan-2(5H)-one with 3-bromo-1,1-dichloroprop-1-ene (cf. preparation example, method 2, example (3)). PCT/EP2007/002386 also describes the preparation of 4-aminobut-2-enolides, for example of 4-[[(6-chloropyridin-3-yl)methyl](3,3-dichloroprop-2-en-1-yl)amino]furan-2(5H)-one by reaction of 4-[(2-fluoroethyl)amino]furan-2(5H)-one with 2-chloro-5-chloromethylpyridine (cf. preparation examples, method 3, example (4)). The reactions are preferably conducted with hydrides of lithium or of sodium. These substrates are generally costly and can simultaneously be handled only with difficulty for safety reasons. In the process according to the invention, in contrast, inexpensive bases which are simpler to handle from a safety point of view, for example NaOH, are used.